It is known to produce cables by taking a central conductor, for example, of stranded copper and extruding thereupon an electrostatic shielding followed by a layer of insulation. The electrostatic shielding is made of a semiconducting material and forms a smooth surface which, if the conductor is used to carry high voltage will obviate corona discharge from the relatively sharply curved outer circumference of the conductor. Another function of the semiconducting electrostatic shield is to provide a smooth conducting surfce inside the insulation layer to maintain the inner surface of the insulation at constant potential. These two functions minimize electrical erosion of the insulation layer which is of a polymeric material having high dielectric strength.
As a final step, a jacket of polyvinyl chloride composition may be extruded about the insulated cable, the jacket providing moistureproofing and mechanical protection. Intermediate the jacket and insulation layers it is usual to provide ground wires which maintain the exterior of the insulation layer and inner surface of the jacket at a common potential. As a consequence of the semiconductive shielding and ground wire systems, electrical stress across the insulation will be uniform and the possibility of failure of the insulation due to electrical causes is minimized.
The foregoing describes generally the constituents of commercial cable construction. Heretofore, the steps of applying the various concentric layers have been carried out separately, that is to say, discontinuously. Such separate manufacture has required considerable expense, separate storage and handling of the cable at different stages of its manufacture, with the possibility of damage to interior portions of the cable prior to its completion. Furthermore, apparatus which might readily be used during more than one step was unnecessarily duplicated.
It is the object of the present invention to provide an improved method and apparatus for the manufacture of commercial cable by means of a continuous process, which avoids many of the difficulties inherent in prior methods.
It is another object of the present invention to provide a superior method and apparatus for vulcanizing an extruded insulation layer for cooling the cable thereafter.